The present invention generally relates to cellular telephone communications systems, airborne cellular systems, and specifically to an airborne cellular GSM (Global System for Mobile Communications) system that provides service by extending a cellular Um interface.
Use of cellular telephones and other wireless data devices onboard aircraft has been banned by the Federal Communications Commission (FCC) and restricted by the Federal Aviation Administration (FAA). The FCC ban is in place to avoid interference with terrestrial cellular systems while an aircraft flies over a cellular network. The FAA regulations restrict the use of cell phones on an aircraft to ensure against interference to onboard communications and navigation equipment. In mid-December, 2004, the FCC announced that it is going examine relaxing its ban on cell phone use in aircraft. A relaxation in the FCC rules will still be subject to the rules and policies of the FAA and aircraft operators.
The FCC is proposing to permit airborne use of cell phones and other wireless devices at the devices lowest power settings under control of a picocell located on the aircraft and only if such operation does not interfere with terrestrial cellular systems. In small cell phone networks picocells are the smallest of radio cells. Picocells often extend to just a few hundred meters in diameter in ground applications. Picocells are generally used to fill in areas of poor coverage or provide coverage in remote locations where there is no standard cellular service available. Onboard an aircraft a cell phone user makes a call that goes to the picocell. The picocell then communicates from the aircraft using a transceiver to a ground station or to a satellite and from the satellite to a ground station and finally to connect to a public switched telephone network (PSTN).
An onboard picocell presents multiple problems related to connection to a ground-based cellular infrastructure. These problems have been and are being addressed by various companies and organizations.
One approach to offering cellular GSM service on aircraft is to place a picocell or BTS (base transceiver station) onboard. The BTS must be connected to a base station controller (BSC) on the ground. To offer onboard cell phone service worldwide, this air-ground link is typically supplied through satellite communications systems (SATCOM) such as Globalstar and Inmarsat. A normal BTS to BSC interface (A-bis interface) requires a T1 line. A T1 line is a high bandwidth telephone line that can handle 24 voice or data channels at 64 kilobits per second, over two twisted pair wires. A T1 line is capable of sending and receiving very large text files, graphics, sounds, and databases very quickly. A dedicated T1 over an existing SATCOM system is inefficient and very expensive.
One solution to the problem is to use a gateway onboard the aircraft to reduce the information on the A-bis connected to the onboard BTS, passing the reduced data over the satellite link, connecting to a second gateway on the ground to regenerate the A-bis before connecting to the ground-based BSC.
Another problem to be solved is the apparent need for an onboard BTS to roam from a BSC at one ground station to that at another. This is the case given the satellite serving the air-ground link must change during a flight, which may occur on flights between the U.S. and Europe for example (depending on the satellite constellation providing the link). Although the GSM standard allows mobiles to roam from one BTS to another, it does not allow for a BTS to roam from one BSC to another. Other approaches seen involve placing more capability onboard, such as including a BSC along with the BTS, or even portions of an MSC (mobile switching center).
Current solutions proposed by industry players only offer partial performance solutions. There is a need for seamless in-flight cell phone coverage by offering a roaming capability. Furthermore, a need exists for an in-flight cell phone system capable of providing needed cell phone performance at lowest costs.